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Abstract:

Provided is an inkjet ink comprising at least water, a pigment, a resin,
a water-soluble organic solvent, and a surfactant, wherein the resin
comprises a copolymer resin synthesized from a monomer that includes a
(meth)acrylic monomer, and the copolymer resin has an acid value of not
less than 50 mgKOH/g and not more than 200 mgKOH/g, a glass transition
temperature (Tg) not less than 20 and not more than 100° C., and a
weight-average molecular weight (Mw) not less than 20,000 and not more
than 100,000, and as a result, a high-quality image is formed in that
abrasion resistance and adhesiveness on a non-absorptive recording medium
are high, glossiness is high, and there is no ink mixing, and ejection
performance is good and recovery through maintenance is excellent. Also
provided is an inkjet recording method that uses the same.

Claims:

1. An inkjet ink comprising at least water, a pigment, a resin, a
water-soluble organic solvent, and a surfactant, wherein the resin
comprises a copolymer resin synthesized from a monomer that includes a
(meth)acrylic monomer, and the copolymer resin has an acid value of not
less than 50 mgKOH/g and not more than 200 mgKOH/g, a glass transition
temperature (Tg) not less than 20.degree. C. and not more than
100.degree. C., and a weight-average molecular weight (Mw) not less than
20,000 and not more than 100,000.

2. The inkjet ink of claim 1, wherein the copolymer resin is synthesized
from a composition that includes monomer of (a) methyl methacrylate, (b)
acrylic ester or methacrylic alkyl ester having alkyl group of 2-8
carbons and (c) acidic monomer.

3. The inkjet ink of claim 1, wherein the copolymer resin is synthesized
from a composition comprising (a) methyl methacrylate, (b) acrylic ester
and (c) acidic monomer, and the copolymer resin has an acid value of not
less than 50 mgKOH/g and not more than 150 mgKOH/g, and a glass
transition temperature (Tg) not less than 30.degree. C. and not more than
100.degree. C.

4. The inkjet ink of claim 1, wherein the copolymer resin comprising a
copolymer resin synthesized from (a) methyl methacrylate, (b) acrylic
ester and (c) acidic monomer, and the copolymer resin has an acid value
of not less than 50 mgKOH/g and not more than 150 mgKOH/g, and a glass
transition temperature (Tg) not less than 30.degree. C. and not more than
100.degree. C.

5. The inkjet ink of claim 2, wherein the acrylic ester has not less than
5 and not more than 14 carbons, and the acidic monomer is acrylic acid or
methacrylic acid.

6. The inkjet ink of claim 2, wherein the acrylic ester is at least one
selected from a group of ethyl acrylate, n-butyl acrylate, i-butyl
acrylate or 2-ethylhexyl acrylate.

7. The inkjet ink of claim 2, wherein a mass of the acrylic ester is not
less than 5% by mass and not more than 45% by mass based on a total mass
of raw monomers for synthesizing the copolymer resin.

8. The inkjet ink of claim 1, wherein a ratio of mass of the copolymer
resin to the pigment is not less than 1 and not more than 20.

9. The inkjet ink of claim 1 comprising amines having boiling point of
not less than 100.degree. C. and not more than 200.degree. C. in an
amount of not less than 0.2% by mass and not more than 2% by mass.

11. The inkjet ink of claim 2, wherein the methacrylic alkyl ester having
alkyl group of 2-8 carbons is at least one selected from a group of ethyl
methacrylate, n-butyl methacrylate, i-butyl methacrylate, t-butyl
methacrylate and 2-ethylhexyl methacrylate.

12. The inkjet ink of claim 10, wherein a total mass of the methacrylic
alkyl ester having alkyl group of 2-8 carbons and methyl methacrylate is
not less than 80% by mass and less than 90% by mass based on a total mass
of the copolymer resin.

13. The inkjet ink of claim 10 comprising a water soluble alkanol amine
in an amount of not less than 0.3% by mass and not more than 2.0% by
mass.

14. The inkjet ink of claim 10, wherein a monomer composition for
synthesizing the copolymer resin has a total mass of methacrylic alkyl
ester having alkyl group of 2-8 carbons, methyl methacrylate, and acidic
monomer in an amount of not less than 80% by mass and not more than 100%
by mass based on a total mass of the copolymer resin.

15. The inkjet ink of claim 1, wherein the surfactant is silicone based
or fluorine based surfactant.

16. An inkjet recording method comprising steps of: printing the inkjet
ink of claim 1 onto a non-water-absorption recording medium heated at not
less than 35.degree. C. and less than 55.degree. C., and heat-drying the
printed inkjet ink at not less than 55.degree. C. and not more than
90.degree. C.

Description:

FIELD OF THE INVENTION

[0001] The present invention relates to a water-based inkjet recording ink
and an inkjet recording method, specifically relates to an inkjet
recording ink and an inkjet recording method which can be recorded on a
non-absorptive recording medium.

TECHNICAL BACKGROUND

[0002] In recent years, there have been developed ink-jet inks for
industrial use which can be printed directly on a non-absorptive medium
such as a polyvinylchloride sheet (also referred to as a non-absorptive
recording medium or a hydrophobic recording medium). Examples of these
ink-jet inks are cited as: a solvent ink in which an organic solvent is
used as a vehicle of an ink; and a UV ink containing a polymerizable
monomer as a primary component of an ink. A solvent ink is dried by
evaporating its solvent to the air, as a result, a solvent ink has a
problem of emitting a large amount of VOC (Volatile Organic Compound),
which becomes a social problem in recent years. There are other concerns
for a worker about an odor or an effect for the safety. Therefore, it is
required to provide equipment for making sufficient ventilation. A UV ink
is made harden immediately after printing, therefore, an emission of VOC
is close to zero, but many monomers to be used in the ink may have
problem of skin sensitization. Further, there are requirement of
incorporating an expensive UV light source into a printer, and it cannot
be used for printers for every field. Moreover, when printed on a glossy
type sheet, these inks cause extremely lusterless at the ink deposited
portion, resulting in being difficult to have an image having excellent
image quality.

[0003] In the above-described background, there has been developed an ink
which can be printed also directly to a non-water absorptive recording
medium, by using a water-based ink containing water as a major component
which has been widely used in homes and has reduced effects on the
environment.

[0004] There has been proposed a water-based ink containing a water
miscible solvent selected from glycols and glycol ethers in Patent
Document 1. Moreover, there has been proposed an ink containing a graft
co-polymer binder which contains a hydrophobic backbone and a non-ionic
and hydrophilic side-chain, wherein the graft co-polymer binder is
soluble in a water-based vehicle and is not soluble in water. However,
the inventors of the present invention studied the method disclosed in.
Patent Document 1 and found that the method disclosed in Patent Document
1 resulted in insufficient image qualities such as an ink mixing caused
by coalescence of ink droplets deposited adjacently on a non-absorptive
recording medium and a patchy pattern caused by gathering droplets at
solid image having uniform medium density or color bleeding in
multi-color printing (phenomenon in which ink droplets are mixed and a
smear is produced in the boundary area of the image having a different
color). Further, durability of the obtained image was also insufficient.
Further, ejection failure was found when continuously using inkjet head
in the method of Patent Document 1. Generally, ejection failure can be
recovered by maintenance with some frequency. However, recovery through
maintenance in the method of Patent Document 1 did not achieve
satisfactory level.

[0005] Patent Document 2 discloses that styrene-acrylic acid copolymer is
added to an ink. Copolymerizable polymer containing styrene enables to
have high glossiness. However, on the other hand, it often tends to
reduce abrasion resistance and adhesiveness in case of an image directly
printed on a hydrophobic substrate.

[0006] Copolymer between styrene and α-methyl styrene and acidic
monomer such as acrylic acid is commercially available. When employing
this copolymer to an inkjet ink, it enables to have high glossiness,
however, it exhibits extremely bad abrasion resistance and adhesiveness.
Even though reasons are not understood in detail, it is supposed that
since styrene has poor adhesiveness to substrate or the film is too hard
and lack of flexibility, whereby crack or peel causes in the film because
of inability to follow to distortion of soft vinyl chloride.

[0007] On the other hand, all acryl type polymers without including
styrene are also commercially available, in which methyl methacrylate is
used instead of styrene. However, this type without including styrene has
issue which tends to decrease glossiness of an image.

[0008] Patent Document 3 discloses that water-soluble acryl resin without
containing styrene is added to an ink. By using a resin coated pigment as
pigment dispersion, this ink enables to have a certain extent of image
durability (abrasion resistance), even when directly printed on vinyl
chloride substrate. However, in the market, durability against further
friction is required and this ink does not enable to have enough abrasion
resistance for further friction as it is now.

[0012] In view of the foregoing, the present invention was achieved. An
object of the present invention is to provide an environmentally-friendly
water-based ink containing water as a major component which exhibits high
abrasion resistance and adhesiveness to a non-absorptive medium, high
glossiness and a high-quality image without ink mixing. Also disclosed is
to provide an inkjet ink having excellent ejection stability and recovery
through maintenance and an inkjet recording method by using the same.

Means to Solve the Problems

[0013] The above object has been attained by the following constitutions:

1. An inkjet ink comprising at least water, a pigment, a resin, a
water-soluble organic solvent, and a surfactant, wherein the resin
comprises a copolymer resin synthesized from a monomer that includes a
(meth)acrylic monomer, and the copolymer resin has an acid value of not
less than 50 mgKOH/g and not more than 200 mgKOH/g, a glass transition
temperature (Tg) not less than 20° C. and not more than
100° C., and a weight-average molecular weight (Mw) not less than
20,000 and not more than 100,000. 2. The inkjet ink of item 1, wherein
the copolymer resin is synthesized from a composition that includes
monomer of (a) methyl methacrylate, (b) acrylic ester or methacrylic
alkyl ester having alkyl group of 2-8 carbons and (c) acidic monomer. 3.
The inkjet ink of item 1 or 2, wherein the copolymer resin is synthesized
from a composition comprising (a) methyl methacrylate, (b) acrylic ester
and (c) acidic monomer, and the copolymer resin has an acid value of not
less than 50 mgKOH/g and not more than 150 mgKOH/g, and a glass
transition temperature (Tg) not less than 30° C. and not more than
100° C. 4. The inkjet ink of item 1 or 2, wherein the copolymer
resin comprising a copolymer resin synthesized from (a) methyl
methacrylate, (b) acrylic ester and (c) acidic monomer, and the copolymer
resin has an acid value of not less than 50 mgKOH/g and not more than 150
mgKOH/g, and a glass transition temperature (Tg) not less than 30°
C. and not more than 100° C. 5. The inkjet ink of any one of items
2 to 4, wherein the acrylic ester has not less than 5 and not more than
14 carbons, and the acidic monomer is acrylic acid or methacrylic acid.
6. The inkjet ink of any one of items 2 to 5, wherein the acrylic ester
is at least one selected from a group of ethyl acrylate, n-butyl
acrylate, i-butyl acrylate or 2-ethylhexyl acrylate. 7. The inkjet ink of
any one of items 2 to 6, wherein a mass of the acrylic ester is not less
than 5% by mass and not more than 45% by mass based on a total mass of
raw monomers for synthesizing the copolymer resin. 8. The inkjet ink of
any one of items 1 to 7, wherein a ratio of mass of the copolymer resin
to the pigment is not less than 1 and not more than 20. 9. The inkjet ink
of any one of items 1 to 8 comprising amines having boiling point of not
less than 100° C. and not more than 200° C. in an amount of
not less than 0.2% by mass and not more than 2% by mass. 10. The inkjet
ink of item 1, wherein the copolymer resin is synthesized from a
composition comprising (a) methyl methacrylate, (b) methacrylic alkyl
ester having alkyl group of 2-8 carbons and (c) acidic monomer. 11. The
inkjet ink of item 2 or 10, wherein the methacrylic alkyl ester having
alkyl group of 2-8 carbons is at least one selected from a group of ethyl
methacrylate, n-butyl methacrylate, i-butyl methacrylate, t-butyl
methacrylate and 2-ethylhexyl methacrylate. 12. The inkjet ink of item 10
or 11, wherein a total mass of the methacrylic alkyl ester having alkyl
group of 2-8 carbons and methyl methacrylate is not less than 80% by mass
and less than 90% by mass based on a total mass of the copolymer resin.
13. The inkjet ink of any one of items 10 to 12 comprising a water
soluble alkanol amine in an amount of not less than 0.3% by mass and not
more than 2.0% by mass. 14. The inkjet ink of any one of items 10 to 13,
wherein a monomer composition for synthesizing the copolymer resin has a
total mass of methacrylic alkyl ester having alkyl group of 2-8 carbons,
methyl methacrylate, and acidic monomer in an amount of not less than 80%
by mass and not more than 100% by mass based on a total mass of the
copolymer resin. 15. The inkjet ink of any one of items 1 to 14, wherein
the surfactant is silicone based or fluorine based surfactant. 16. An
inkjet recording method comprising steps of printing the inkjet ink of
any one of items 1 to 15 onto a non-absorptive recording medium heated at
not less than 35° C. and less than 55° C., and heat-drying
the printed inkjet ink at not less than 55° C. and not more than
90° C.

Effects of the Invention

[0014] The present invention made it possible to provide inkjet ink which
exhibits high abrasion resistance and adhesiveness to a non-absorptive
medium, high glossiness and a high-quality image without ink mixing as
well as having excellent ejection stability and head recovery through
maintenance and an inkjet recording method by using the same.

PREFERRED EMBODIMENT OF THE INVENTION

[0015] An optimal embodiment to practice the present invention will now be
detailed.

[0016] The inventors of the present invention conducted diligent
investigations about a water-based pigment ink so as to form a
high-quality image without ink mixing even printing onto various resin
substrate such as vinyl chloride sheet for sign use or paper substrate
having slow absorption such as printing paper, and an image exhibiting
high glossiness, high abrasion resistance and adhesiveness, as well as
having excellent ejection stability and head recovery through
maintenance.

(Non-Absorptive Recording Medium)

[0017] The non-absorptive recording medium described above is referred to
as a recording medium comprising hydrophobic resin such as vinyl
chloride, PET, polypropylene, polyethylene, or polycarbonate which
absorbs little water, or a recording medium such as printing paper such
as coat paper which absorbs water to some extent but absorption rate is
too slow to dry the water-based ink in a general inkjet printing process
in an environment of ordinary temperatures and humidity, resulting in
problem for drying.

(Pigment)

[0018] Pigment utilizable in this invention includes an organic and an
inorganic pigment conventionally well known in the art. For example,
listed are an azo pigment such as an azo lake, insoluble azo pigment,
condensed azo pigment and chelate azo pigment; a polycyclic pigment such
as a phthalocyanin pigment, perylene and perylene pigment, anthraquinone
pigment, quinacridone pigment, dioxadine pigment, thioindigo pigment,
isoindolinone pigment and quinophthalone pigment; a dye lake such as a
basic dye type lake, and acidic dye type lake; an organic pigment such as
a nitro pigment, nitroso pigment, aniline black and daylight fluorescent
pigment; and an inorganic pigment such as a carbon black.

[0019] As specific pigments which are preferably usable, the following
pigments are listed:

[0023] So as to maintain stable dispersion of above pigment in a
water-based ink, various treatments are carried out to prepare pigment
dispersion.

[0024] The dispersion may be any one provided being stably dispersible in
a water phase and can be selected from such as a pigment dispersion in
which a pigment is dispersed by polymer resin, capsule pigment in which a
pigment is covered with a water insoluble resin, self-dispersible pigment
in which the surface of a pigment is modified to be dispersible without a
dispersion resin.

[0026] Further, a pigment may be dispersed with the copolymer resin
described above as dispersing resin of pigment.

[0027] As for a dispersion method of a pigment, various types such as a
ball mill, a sand mill, an atliter, a roll mill, an agitator, a Henschel
mixer, a colloidal mill, a ultrasonic homogenizer, a pearl mill, a
wet-type jet mill and a paint shaker can be utilized.

[0028] In this invention, it is also preferable to utilize a centrifugal
separator or to utilize a filter for elimination of coarse particles in
pigment dispersion.

[0029] Further, in the case of utilizing capsule pigment in which a
pigment is covered with a water-insoluble resin, a water-insoluble resin
refers to a resin which is insoluble in water in a range of weak acidic
to weak basic, and preferably the solubility of which against aqueous
solution of pH 4-10 is less than 2%.

[0030] Such a resin includes each resin of such as an acryl type, a
styrene-acryl type, an acrylonitrile-acryl type, a vinyl acetate type, a
vinyl acetate-acryl type, a vinyl acetate-vinyl chloride type, a
polyurethane type, a silicone-acryl type, an acryl silicone type, a
polyester type and an epoxy type.

[0031] The dispersing resin or the water-non-soluble resin preferably has
a weight-average molecular weight of from 3,000 to 500,000, more
preferably from 7,000 to 200,000.

[0032] The dispersing resin or the water-non-soluble resin preferably has
a glass transition temperature (Tg) from about -30° C. to
100° C., more preferably from about -10° C. to 80°
C.

[0033] Mass ratio of pigment to resin for dispersing pigment is preferably
selected in the range of not less than 100/150 and not more than 100/30,
represented by Pigment/Resin ratio. Specifically, good durability of the
image, ejecting stability and ink storage stability are exhibited in the
range of not less than 100/100 and not more than 100/40.

[0034] The average particle diameter of the pigment particles which are
coated by non-water-soluble resin is preferably about 80 to 150 nm, in
terms of ink storage stability and color forming properties.

[0035] Various conventional methods are applicable as a method for coating
pigment by non-water-soluble resin. Preferable is a method in which
non-water-soluble resin is dissolved in organic solvent such as methyl
ethyl ketone, followed by partially or completely neutralizing acid group
in the resin by base. Then pigment and ion-exchanged water are added and
dispersed. After eliminating organic solvent, water is added to arrange
as appropriate. Or preferable is a method in which pigment is dispersed
by using polymerizable surfactant, and coated while polymerization is
carried out by supplying monomer thereto.

[0036] Further, self-dispersing pigment which is surface treated available
on the market may be applicable. Specific examples of preferable
self-dispersing pigments include CAB-JET 200, CABO-JET 300 (produced by
Cabot Corporation), and BONJET CW1 (produced by Orient Chemical
Industries Co., Ltd).

(Organic Solvent)

[0037] Organic solvent having low surface tension may be preferably added
to the ink of the present invention.

[0038] Addition of organic solvent having low surface tension enables to
inhibit ink mixing more even to a recording medium made of various
hydrophobic resins such as soft vinyl chloride sheet, or a paper support
having low ink absorption such as printing paper, resulting in having
high quality print images. It is considered that the organic solvent
having low surface tension functions to improve ink wettability to vinyl
chloride, as well as the copolymer resin functions to increase ink
viscosity according to drying water in ink.

[0039] Especially glycol ethers or 1,2-alkanediols is preferably added to
the ink used for the present invention. Specifically preferred is to
employ following water-soluble organic solvent (figure in parenthesis
represents surface tension).

[0042] Further, preferred is addition of solvent capable of dissolving,
softening or swelling a recording medium such as vinyl chloride, because
it enables to enhance adhesiveness between vinyl chloride and the
copolymer resin, whereby enhances adhesiveness and abrasion resistance of
image.

[0044] Specific examples of preferable cyclic solvent containing a
nitrogen atom is a cyclic amide compound and a 5-7-member ring, and
includes such as 2-pyrrolidone, N-methyl-2-pyrrolidone,
N-ethyl-2-pyrrolidone, 1,3-dimethyl-2-imizolidinone,
ε-caprolactam, methyl caprolactam and 2-azacyclooctanone.

[0045] The cyclic solvent containing a sulfur atom is preferably a
5-7-member ring and includes such as sulforane.

[0046] Specific example of the cyclic ester solvent includes such as
γ-butyllactone and ε-caprolactone; specific example of the
lactic acid ester includes butyl lactate and ethyl lactate.

[0047] Specific example of the alkyleneglycol diether includes
diethyleneglycol diethylether.

[0051] Addition of silicone type or fluorine type surfactant as the
surfactant enables to inhibit ink mixing more even to a recording medium
made of various hydrophobic resins such as soft vinyl chloride sheet, or
a paper support having low ink absorption such as printing paper,
resulting in having high quality print images. The surfactant is employed
preferably in combination with the water-soluble organic solvent having
low surface tension.

[0052] The silicone type surfactant is preferable polyether modified
polysiloxane, and includes KF-351A and KF-642 produced by Shin-Etsu
Chemical Co., Ltd.; and BYK347 and BYK348 produced by BYK-Chemie GmbH.

[0053] Fluorine type surfactant is a surfactant in which a part or all of
hydrogen atoms bonded to carbon atom in hydrophobic group of general
surfactant are replaced by fluorine atom. Of these, fluorine type
surfactant having a perfluoro alkyl group is preferably used.

[0054] Examples of fluorine type surfactant are commercially available as
"Megafac F" from Dainippon Ink and Chemicals, Inc., "Surflon" from Asahi
glass Co., Ltd., "Fluorad FC" from Minnesota Mining and Manufacturing,
"Monflor" from Imperial Chemical Industries, "Zonyls" from E.I. DuPont as
well as "Licowet VPF" from Farbwerke Heochest AG under the trade names.

[0055] According to species of hydrophilic group, fluorine type
surfactants are classified into anionic, cationic and nonionic type
surfactant, and nonionic type surfactant can be preferably employed.

[0056] Preferable specific examples of nonionic fluorine type surfactant
include Megafac 144D produced by Dainippon ink, and Surflon S-141
produced by Asahi glass. As amphoteric fluorine type surfactant, listed
are Surflon S-131 and 132 produced by Asahi glass.

[0057] Silicone type or fluorine type surfactant may be employed in
combination with following surfactants.

[0059] By employing the water-based inkjet ink of the present invention,
it enables to print a high-quality image without ink mixing on the
non-absorptive recording medium, whereby to form excellent image having
high glossiness, high abrasion resistance and adhesiveness.

[0060] In terms of forming higher quality image having abrasion resistance
and adhesiveness and corresponding to faster printing conditions,
printing is preferably performed while heating a recording medium at not
less than 35° C. and less than 55° C. At more than
55° C., it caused problems such that a recording medium made of
vinyl chloride tends to be deformed to be undulated and ejection becomes
unstable due to drying ink at head.

[0061] Further, it is more preferable that print is heat-dried at not less
than 55° C. and less than 90° C. after printing while
heating, because heating after printing enables to accelerate drying as
well as enhancing adhesiveness between the copolymer resin and the
non-absorptive recording medium.

[0062] This effect is specifically obtained when printing onto the
non-absorptive recording medium. When an organic solvent has relatively
higher boiling point than water in an ink, sometimes the organic solvent
cannot be dried and remains on a surface of printing side. The residual
solvent exists in combination with copolymer resin on the printing
surface and sometimes causes insufficient film-forming and curing of the
copolymer resin, whereby resulting in decreasing abrasion resistance or
adhesiveness. When further heat-drying is carried out after printing, the
residual solvent on the non-absorptive recording medium can be
eliminated, whereby abrasion resistance and adhesiveness can be enhanced
more.

[0063] As for the heating temperature, heating is preferably performed at
not less than 55° C. in terms of accelerating a elimination of
residual solvent and at less than 90° C. in terms of inhibiting
deformation of a recording medium by heating.

[0064] When using an absorptive recording medium such as paper, a residual
solvent can be absorbed into the recording medium and does not remain at
the surface of printing side, whereby above phenomena does not occur.

[0065] As for a specific heat-drying method, listed are a method in which
heating is performed by heater after printing from a back side of a
recording medium, a method in which heating is performed by a warm air
blow from a printing side of a recording medium, or a method in which
heating of a recording medium is performed by radiation heat of infrared
ray by use of such as a halogen lamp.

(Copolymer Resin)

[0066] The copolymer resin functions as binder of pigment colorant to have
adhesiveness to a non-absorptive recording medium such as vinyl chloride
and also functions to enhance abrasion resistance of layer.

[0067] The copolymer resin has to function to form an image having high
glossiness and high optical density. Therefore, the copolymer itself is
required to have high transparency in the layer and miscibility with
pigment or pigment dispersing resin.

[0068] Further, printed image having high quality without ink mixing is
necessary even when printed onto various non-absorptive recording medium
such as vinyl chloride sheet. The inventor of the present invention found
that two factors were necessary to reduce ink mixing. One factor is a
wettability of ink onto a substrate. Another one is ease of viscosity
increase after ink depositing onto a substrate. Resin for ink must not
drastically degrade a wettability of ink and must increase viscosity in
drying process as soon as possible after ink deposition.

[0069] Further, it is required that an addition of resin to an ink must
not deteriorate an ejection performance. Even when continuous print or
continuous pose causes to deteriorate an ejection performance, it must be
easily recovered to the initial ejection state by maintenance.

[0070] As described above, for addition of resin to ink, the resin has to
be designed and selected to have enough effects in various purposes.

[0071] In view of the foregoing, the inventors of the present invention
conducted diligent investigations with respect to various resins. As a
result, the following was discovered, and the present invention was
achieved. That is, an ink having acrylic type copolymer resin with low
acid value exhibits excellent abrasion resistance and the copolymer resin
having relatively high molecular weight exhibits less ink mixing.

[0072] As is well known in the art, since acryl resin can be selected and
designed freely from extremely various type of monomers and be easily
polymerized and produced in low cost, it is suitable to the present
invention.

[0073] Acryl resin available in the market includes water-dispersion type
acryl emulsion and water soluble resin. Emulsion type has advantage
generally to have higher molecular weight than water soluble type,
resulting in forming film having enhanced strength. On the contrary, once
film is dried, it cannot be dissolved into water. Therefore, since dried
ink on the head cannot be eliminated by dissolving and has to be
physically scraped away, thereby maintenance tends to become difficult.
Therefore, the copolymer resin is preferable water soluble resin.

[0074] The (meth)acryl based monomer refers to a monomer comprising
(meth)acrylic acidic monomer and a derivatives thereof. Specific examples
of (meth)acryl based monomer include (meth)acrylic acid, (meth)acrylic
acid esters, and (meth)acrylamides. Of these, (meth)acrylic acid and
(meth)acrylic acid esters are preferably used.

[0075] The copolymer resin is synthesized from a monomer that includes a
(meth)acrylic based monomer, and the copolymer resin has an acid value of
not less than 50 mgKOH/g and not more than 200 mgKOH/g, a glass
transition temperature (Tg) of not less than 20° C. and not more
than 100° C., and a weight-average molecular weight (Mw) of not
less than 20,000 and not more than 100,000.

[0079] One of features of the copolymer resin related to the present
invention is to have an acid value of not less than 50 mgKOH/g and not
more than 200 mgKOH/g. When a resin has acid value of not less than 50
mgKOH/g, since a resin can be eliminated by resolution or physical
rubbing process even when drying causes at nozzle of inkjet head, thereby
excellent maintenance property can be applied. Further, in case of acid
value being not more than 200 mgKOH/g, excellent abrasion resistance,
adhesiveness and glossiness can be obtained.

[0080] Further, one of features of the copolymer resin related to the
present invention is to have a glass transition temperature (Tg) of not
less than 20° C. and not more than 100° C. In case of Tg
being not less than 20° C., excellent abrasion resistance can be
obtained as well as blocking is inhibited. In case of Tg being less than
100° C., excellent adhesiveness of image film can be maintained.

[0081] One of features of the copolymer resin related to the present
invention is to have a weight-average molecular weight (Mw) not less than
20,000 and not more than 100,000. In case of weight-average molecular
weight of the copolymer resin being not less than 20,000, enough abrasion
resistance can be obtained. In case of weight-average molecular weight
being not more than 100,000, excellent ejection stability and maintenance
property can be obtained.

[0082] Content of the copolymer resin is preferable not less than 1% by
mass and not more than 15% by mass based on a total ink mass, more
preferable not less than 2% by mass and not more than 10% by mass.

[0083] In the case when the copolymer resin has a mass ratio to pigment
solid being more than 1 time, excellent abrasion resistance, adhesiveness
and glossiness of the image can be obtained. In case of a mass ratio
being not more than 20 times, ejection stability and maintenance property
cannot be become impaired. More preferably, a mass ratio is not less than
1 time and not more than 10 times.

(Neutralization of Resin)

[0084] In the copolymer resin, a portion corresponding to acidic monomer
may be neutralized partially or completely by using base. Bases for
neutralizing acidic group include a base containing alkali metal (for
example, NaOH and KOH), amines (for example, alkanol amines and alkyl
amines) or ammonia.

[0085] In view of enhancing durability of image and inhibiting ink mixing,
it is preferable to neutralize by using amines having boiling point of
not more than 200° C.

[0086] Next, a case of using acrylic acid ester as monomer (b) of the
copolymer resin will be detailed.

[0087] Methacrylic acid is preferably added in an amount of 15-90% by mass
based on a mass of total monomers comprising raw materials of the
copolymer resin.

[0088] As for the acrylic acid ester, one or a plurality species of
monomers may be copolymerized. Carbon numbers of acrylic acid ester are
preferably not less than 5. Since carbon chain in ester portion is long,
thereby copolymer resin in the ink and the recording medium made of
hydrophobic resin are closely adhered, resulting in enhancing abrasion
resistance or adhesiveness. Further, carbon numbers of acrylic acid ester
are preferably not more than 14, in terms of ejection stability.

[0090] Further, a mass ratio of the acrylic acid ester is preferable not
less than 5% and not more than 45% based on a mass of total monomers
comprising the copolymer resin. In case of the mass ratio of the acrylic
acid ester being not less than 5%, adhesiveness to a recording medium
made of hydrophobic resin can be high. In case of the mass ratio of the
acrylic acid ester being not more than 45%, excellent ejection can be
obtained.

[0091] The copolymer resin preferably has an acid value of not less than
50 mgKOH/g and not more than 150 mgKOH/g.

[0092] The inventor of the present invention conducted detail
investigations about various resins and found that there is close
relationship between acid value of resin and abrasion resistance or
adhesiveness such that the lower is the acid value of resin, the higher
are abrasion resistance and adhesiveness. It is supposed that in case of
acid value of resin being high, the resin becomes hydrophilic and
affinity to the hydrophobic substrate becomes low, resulting in being
difficult to adhere between the resin and the substrate. On the contrary,
in ease of acid value of resin being low, the resin becomes hydrophobic
and affinity to the hydrophobic substrate becomes high, resulting in
enhancing to adhere between the resin and the substrate.

[0093] Further, there is relationship between acid value of resin and
ejection property of ink or maintenance property. In case of excessive
low acid value, since dried ink on the head cannot be eliminated by
dissolving and has to be physically scraped away, thereby maintenance
tends to become difficult.

[0094] Further, acid value of resin has effect to ink mixing or
glossiness. In case of excessive high acid value, it tends to result in
deterioration of ink mixing or glossiness.

[0095] From above, an acid value of the copolymer resin is preferable not
less than 50 mgKOH/g and not more than 150 mgKOH/g, more preferable not
less than 60 mgKOH/g and not more than 100 mgKOH/g.

[0096] Further, the copolymer resin preferably has a glass transition
temperature (Tg) of not less than 30° C. and not more than
100° C. In case of Tg being less than 30° C., abrasion
resistance is insufficient and sometimes blocking may occur. In case of
Tg being higher than 100° C., abrasion resistance becomes
extremely deteriorated. It is supposed that a film after drying becomes
too hard to be brittle. Glass transition temperature (Tg) of the
copolymer resin can be arranged by species and composition ratio of
monomers used for copolymerization.

[0097] Weight-average molecular weight (Mw) of the copolymer resin is not
less than 20,000 and not more than 100,000. In case of weight-average
molecular weight being not less than 20,000, abrasion resistance becomes
better. In case of weight-average molecular weight being not more than
100,000, ink ejection or maintenance property becomes excellent. More
preferable weight-average molecular weight of the copolymer resin is not
less than 25,000 and not more than 70,000.

[0098] Further, weight-average molecular weight of the copolymer resin has
effect to ink mixing. In case of excessive small, ink mixing performance
becomes worse. Ink mixing refers to a phenomenon in which adjacently
deposited ink droplets contact each other to mix during the time from
deposition to drying, resulting in deforming dot shapes or mixing colors
which causes deterioration of image quality. In order to inhibit this ink
mixing, it is necessary to inhibit mixing between adjacently deposited
ink droplets by quickly drying after ink deposition onto the substrate.
The inventors of the present invention consider that weight average
molecular weight relates to an increase of viscosity during drying
process after ink deposition. The larger is a weight average molecular
weight, viscosity may increase more easily, whereby ink mixing can be
inhibited. In case of weight average molecular weight being not less than
20,000, less deterioration of images occurs.

[0099] Weight average molecular weight of the copolymer resin can be
arranged by polymerization reaction conditions such as concentration of
monomers or content of initiator. For example, weight average molecular
weight can be larger by using higher concentrated monomer. Or weight
average molecular weight can be smaller by increasing a content of
initiator.

[0100] The copolymer resin may be added before dispersing pigment, or
after dispersing process. Of these, addition after dispersing process is
preferred.

[0101] The copolymer resin is preferably added 1%-15% by mass in ink, more
preferably 3%-10% by mass.

[0102] In the inkjet ink of the present invention, a resin other than the
copolymer resin may be employed in combination. Preferable content ratio
of the copolymer resin to total resin containing in the ink is not less
than 25% by mass and not more than 100% by mass.

(Neutralization of Resin)

[0103] Specifically preferred is a neutralization by using amines having
boiling point of not less than 100° C. and not more than
200° C., in terms of dissolving the copolymer resin into the ink,
enhancing image durability, or inhibiting ink mixing. Neutralization by
N,N-dimethylamino ethanol and 2-amino-2-methylpropanol are preferable, in
terms of ejection stability.

[0104] Content of base for neutralization depends on an amount of acidic
monomer containing in the copolymer resin. In case of content being
excessive small, an effect of neutralization of the copolymer resin
cannot be obtained. In case of content being excessive large, it causes
problems such as water resistance, color change or odor of the image.
Therefore, preferable content is not less than 0.2% and not more than 2%
in ink.

[0105] Subsequently, the case of using alkyl methacrylate ester having 2-8
carbons in alkyl group as the monomer (b) in the copolymer resin will now
be specifically described

[0106] One of features of the copolymer resin related to the present
invention is to have a total mass of methacrylic alkyl ester having alkyl
group of 2-8 carbons, methyl methacrylate, and acidic monomer in an
amount of not less than 80% by mass and not more than 100% by mass based
on a total mass of the copolymer resin. In case of total mass being not
less than 80% by mass, intended abrasion resistance and adhesiveness can
be obtained. Although the factor is not cleared yet, it is supposed that
copolymer resin comprising three component of methacrylic alkyl ester
having alkyl group of 28 carbons, methyl methacrylate and acidic monomer
can co-exist stably with various water based pigment dispersion and can
form highly uniform coating layer, resulting in exhibiting less
glossiness deterioration.

[0107] Further, since methacrylic alkyl ester having alkyl group of 2-8
carbons can enhance adhesiveness to the substrate, thereby high abrasion
resistance and adhesiveness can be exhibited. On the contrary, when large
amount of styrene or styrene having substituent is added, adhesiveness to
the substrate decreases and abrasion resistance and adhesiveness
extremely decreases. Further, in case of carbon number of alkyl group in
methacrylic alkyl ester being not less than 9, ejection stability becomes
deteriorated. Therefore these conditions are undesired.

[0108] One of features of the copolymer resin related to the present
invention is to have a weight-average molecular weight (Mw) not less than
20,000 and not more than 100,000, preferably less than 100,000. In view
of enhancing all of abrasion resistance, ejection stability and
maintenance property, a weight-average molecular weight is preferable not
less than 30,000 and less than 60,000.

[0109] The copolymer resin related to the present invention will now be
detailed.

[0110] As for copolymerizable monomer in the copolymer resin related to
the present invention, other copolymerizable monomer may be appropriately
added in the range of less than 20% based on the total copolymer resin
composition comprising monomers of methacrylic alkyl ester having alkyl
group of 2-8 carbons, methyl methacrylate, and acidic monomer.

[0111] In the copolymer resin related to the present invention, it is
preferable to copolymerize methyl methacrylate in the range of from not
less than 20% by mass to not more than 50% by mass based on the total
mass of the copolymer resin, in view of enhancing abrasion resistance and
adhesiveness.

[0112] Further, methacrylic alkyl ester having alkyl group of 2-8 carbons
is preferably at least one selected from a group of ethyl methacrylate,
n-butyl methacrylate, i-butyl methacrylate, t-butyl methacrylate and
2-ethylhexyl methacrylate. Further, one or a plurality of methacrylic
alkyl esters having alkyl group of 2-8 carbons may be copolymerized. Of
these, at least one of ethyl methacrylate and n-butyl methacrylate is
preferably copolymerized, in view of enhancing ejection stability.

[0113] Methacrylic alkyl ester having alkyl group of 2-8 carbons is
preferably copolymerized in the range of from not less than 25% by mass
to less than 70% by mass based on the total mass of the copolymer resin,
in view of enhancing abrasion resistance and adhesiveness.

[0114] In the copolymer resin related to the present invention,
methacrylic alkyl ester having alkyl group of 2-8 carbons is preferably
copolymerized in the range of from not less than 80% by mass to less than
90% by mass based on the total mass of the copolymer resin, in view of
forming film having excellent abrasion resistance and adhesiveness.

[0115] Further, in the copolymer resin related to the present invention,
employable may be a monomer in which a portion corresponding to acidic
monomer may be neutralized partially or completely by using alkali. Of
these, acidic monomer neutralized by ammonia is preferable, since drying
is fast after printing, thereby printed matter can be stacked immediately
after printing, as well as preventing ink mixing. As for a counter salt,
N-dimethylaminoethanol, 2-amino-2-methylpropanol and N-methylaminoethanol
is preferably employed, in view of stable ejection.

[0116] The copolymer resin related to the present invention can be
provided according to a conventional method for synthesizing polymer.
Example of synthesizing method of the copolymer resin relating to the
present invention will be described below.

[0117] Into 500 ml of a four-neck flask provided with a mechanical
stirrer, a nitrogen gas directing tube, a condenser, and a dropping
funnel, were loaded 185 g of isopropyl alcohol, and heated to reflux,
while bubbling a nitrogen gas. Into the dropping funnel, 46 g of methyl
methacrylate, 39 g of n-butyl methacrylate, 15 g of methacrylic acid and
0.5 g of azobisisobutyronitrile (AIBN) as an initiator were dissolved in
mixture, and the mixture was dropped for about 2 hours in a state of
heated to reflux. After finishing dropping, further heated to reflux for
6 hours, and isopropyl alcohol solution containing 0.05 g of AIBN was
dropped for 15 minutes. Thereafter, resulting solution was heated to
reflux for further 5 hours.

[0118] After standing to cool the reaction solution, isopropyl alcohol was
distilled away under reducing pressure. Into a residue, 12.4 g of 28%
ammonia water as base for alkali neutralization and 553 g of ion
exchanged water were added and heated, stirred to dissolve, whereby
prepared was an ammonia salt of copolymer resin. Solid content of the
copolymer resin was about 15% by mass. Acid value of the copolymer resin
was determined by acid value measurement according to JIS K-0070 Acid
value measurement by hydrolysis (total acid value measurement) to be 102
mg KOH/g. Glass transition temperature determined by DSC was 79°
C. Weight average molecular weight was determined by GPC measurement to
be 46,000. Measurement method for each property was detailed in the
following Examples.

[0119] The copolymer resin of the present invention is preferably added 1
to 4 times by mass based on a solid content of pigment.

(Pigment Dispersion)

[0120] Any pigment which can be stably dispersible in water may be
employed to the present invention. In case of valuing ink storage
stability, it is preferred to select capsule pigment covered with a
water-insoluble resin.

[0122] As molecular weight of water insoluble resin, resin having weight
average molecular weight of 3,000 to 500,000 are employable, preferable
7,000 to 200,000.

[0123] As Tg of water insoluble resin, resin having -30° C. to
100° C. are employable, preferable -10° C. to 80° C.

[0124] Solution polymerization method or emulsion polymerization method
can be employable as polymerization method. Polymerization may be
preliminarily carried out separately from pigment, or by supplying
monomer into dispersed pigment.

[0125] Various conventional methods can be employed for covering pigment
by water insoluble resin.

Other than phase inversion emulsification method or acid deposition
method, it is preferable to select a method in which pigment is dispersed
by using polymerizable surfactant, and monomer is supplied thereto and
covers while polymerizing.

[0126] As more preferable method for covering pigment by water insoluble
resin, listed is the following method in which water insoluble resin is
dissolved in organic solvent such as methyl ethyl ketone, and acid group
in the resin is neutralized partially or completely by using base. Then,
pigment and ion exchanged water are added and dispersed, followed by
eliminating organic solvent and appropriately adding water to prepare
pigment covered by water insoluble resin.

[0127] A mass ratio of pigment to water insoluble resin can be selected in
the range of from 100/40 to 100/150 by the ratio of Pigment/Water
insoluble resin. Specifically, 100/60 to 100/110 is preferable, in view
of excellent image durability, ejection stability and ink storage.
Average particle size of pigment particle covered by water insoluble
resin is preferable about 80-150 nm, in view of ink storage stability and
coloring property.

(Organic Solvent)

[0128] In the present invention, in view of ink ejection stability from
inkjet head, maintenance property and glossiness of the formed image, as
one of solvents, ink preferably contains water-soluble alkanol amines not
less than 0.30% by mass and not more than 3.0% by mass, more preferably
not less than 0.3% by mass and not more than 1.8% by mass based on total
mass of the ink. Preferably employable water-soluble alkanol amines to
the present invention include N,N-dimethylaminoethanol,
2-amino-2-methylpropanol and N-methylaminoethanol.

EXAMPLES

[0129] Embodiments of the present invention will now be specifically
described with the reference to examples, however the present invention
is not limited thereto. Incidentally, the expression of "part" or "%"
referred to in Examples represents "part by mass" or "% by mass" unless
otherwise specified.

[0130] In examples, acid value, glass transition temperature (Tg) and
weight average molecular weight are determined by the following methods.

(Measurement of Acid Value)

[0131] Into 300 ml of conical flask, 10 g of a resin was weighed and
dissolved in 50 ml of ethanol-benzene (1:2) mixture solution. Then, the
resulting solution was subjected to titration employing phenol phthalein
indicator and a preliminarily standardized 0.01 mol/L potassium hydroxide
ethanol solution. From the amount of potassium hydroxide ethanol solution
used for titration, acid value in terms of mg KOH/g was determined by the
following Calculation formula (I). When the resin could not be dissolved
in 50 ml of ethanol-benzene (1:2) mixture solution, titration was carried
out in the same manner as above, except for selecting either 50 ml of
ethanol or 50 ml of ethanol-pure water (1:1) mixture solution which could
dissolve the resin.

[0135] The measurement is conducted as follows. A resin in an amount of
10.00 mg is precisely weighed to two places of decimals, sealed into an
aluminum pan (KIT NO. 0219-0041) and set into a DSC-7 sample holder. An
empty aluminum pan is used as a reference.

[0136] Temperature is controlled in the range of 0 to 130° C./min
through heating-cooling-heating at a temperature-raising rate of
10° C./min and a temperature-lowering rate of 10° C./min.
Analysis was carried out based on the data of 2nd Heat. Measurement
was conducted under nitrogen gas flow.

[0137] An extension line from the base-line prior to the initial rise of
the first endothermic peak and a tangent line exhibiting the maximum
slope between the initial rise and the peak are drawn and the
intersection of both lines is defined as the glass transition point Tg.

[0148] Into a flask provided with a dropping funnel, a condenser, a
nitrogen gas directing tube, a thermometer, and a mechanical stirrer,
were loaded 64.8 parts of 2-propanoland heated to reflux, while bubbling
a nitrogen gas. Into the dropping funnel, 18.2 parts of methyl
methacrylate, 12.6 parts of n-butyl acrylate, 4.2 parts of methacrylic
acid and 0.2 parts of initiator (AIBN) were dissolved in mixture, and the
mixture was dropped for about 2 hours. Thereafter, resulting solution was
heated to reflux for thither 5 hours. After standing to cool the reaction
solution, 2-propanol was distilled away under reduced pressure to obtain
Copolymer resin P-1.

Synthesis of Copolymer Resins P-2 to P-19

[0149] Copolymer resins P-2 to P-19 were synthesized in the same manner as
Synthesis of Copolymer resin P-1, according to the monomer composition
ratio listed in Table 1. Herein, weight average molecular weights were
arranged by changing amount of initiator.

[0150] Tg, acid value and Mw of the copolymer resins were listed in Table
1. Monomers described by abbreviation in Table 1 were as follows:

[0156] Into ion exchanged water of 66 parts, 12 parts of efka 4570 (solid
content: 60%, produced by EFKA) and 5 parts of diethyleneglycol monobutyl
ether were added and mixed. Into solution, 15 parts of C.I. Pigment Blue
15:3 was added. After pre-mixing, the resulting solution was dispersed by
use of a sand grinder filled at a volume ratio of 50% with zirconia beads
of 0.5 mm, whereby Cyan pigment dispersion having pigment content of 15%
was prepared.

(Preparation of Ink)

Preparation of Ink C-1

[0157] Into 30 parts of ion exchanged water, 5 parts of Copolymer resin
P-1 was loaded. Thereto, loaded was 1.05 times of chemical equivalent
weight of N,N-dimethylaminoethanol based on the chemical equivalent
weight of Copolymer resin P-1 and followed by heating at 60° C.
and mixing to dissolve. After standing to cool the solution, loaded were
diethyleneglycol monobutyl ether, diethyleneglycol monoethyl ether, and
1,3-dimethyl-2-imidazolidinon as organic solvent, so as to arrange ink
content listed in Table 2. Further, loaded was 0.5 parts of KF-351A
(produced by Shin-Etsu Chemical Co., Ltd.) as silicone type surfactant
and mixed. The solution was arranged to be 80 parts in total mass by
adding ion exchanged water. Then, loaded was 20 parts of the cyan pigment
dispersion thereto and mixed, followed by filtering by 0.8 μm filter,
whereby Cyan ink C-1 was prepared.

Preparation of Inks C-2 to C-21

[0158] Inks C-2 to C-21 were prepared in the same manner as Ink C-1 by
employing species and content of copolymer resin, base for
neutralization, organic solvent and surfactant listed in Table 2 and by
arranging content of cyan pigment dispersion so as to have mass ratio of
Copolymer/Pigment listed in Table 2. When base for neutralization was
ammonia, 28% ammonia water was employed. In Table 2, contents of resin in
copolymer resin and organic solvent represents content based on ink (% by
mass).

[0159] Organic solvents, bases for neutralization, and surfactants
described by abbreviation in Table 2 were as follows:

[0169] Each ink prepared above was evaluated according to the following
methods. Herein, surface tension of each ink determined by plate method
was in the range of 26-30 mN/m.

(Image Formation)

[0170] Each ink prepared above was set in one of the inkjet head in an
on-demand type inkjet printer which has 4 lines of piezo type heads where
a nozzle pore size was 28 μm, a driving frequency was 18 kHz, a nozzle
number was 512, an amount of an minimum ink droplet was 14 pl, and a
nozzle density was 180 dpi.

[0171] The printer can warm a medium from the bottom (the opposite side
facing to the head) by a contact type heater at appropriate temperature,
being equipped with a position for ink idle print and a maintenance unit
in a head housing position, and can perform head cleaning in arbitrary
frequency.

[0172] Then, onto recording medium of soft vinyl chloride sheet for
solvent inkjet printer, images with a printing resolution 720
dpi×720 dpi, and solid image by 10 cm×10 cm size with 100%
and 50% Duty were printed to be a recorded image.

[0173] During printing onto the polyvinyl chloride recording medium, back
side of the recording medium was heated by controlling heater to be at
45° C. at a surface temperature of the recording medium when image
was recorded. Herein, the surface temperature of the recording medium was
determined by using Non-contact thermometer (IT-530N type produced by
Horiba Ltd.). After recording, the printed matter was dried for 24 hours
under room temperature (25° C.) and recorded images and printing
performances were evaluated as follows.

<<Evaluation of Recorded Image and Printing Performance>>

(Abrasion Resistance)

[0174] Each of the resulting solid images was scrubbed by dry cotton
(Kanakin No. 3) with load of 300 g. Abrasion resistance was evaluated
based on the following criteria.

[0175] A: No change was observed in image under friction of 50 times or
more.

[0176] B: Slight scratch remained but did not affect image density after
friction of 50 times.

[0177] C: Image density became lowered during friction between 21 and less
than 50 times.

[0178] D: Image density became lowered under friction of less than 20
times.

[0179] Ranks A to C were considered to be practically preferable in above
criteria.

(Evaluation of Adhesiveness)

[0180] Three cm of adhesive cellophane tape was adhered onto each
resulting solid image of 100% Duty. After that, the tape was quickly
peeled from the layer, and the surface state of the image was observed.

[0181] A: No change was observed at the surface state.

[0182] B: Slight trace was remained on the surface.

[0183] C: A part of the image was peeled off and the decrease of color
density was observed.

[0184] D: Most of the image was peeled off and the substrate was observed
as white background.

[0185] Ranks A to C were considered to be practically preferable in above
criteria.

(Evaluation of Glossiness)

[0186] For the solid image portion having 100% Duty of the recorded image,
20° specular glossiness was measured. A declination glossmeter
VGS-10001DP supplied from Nippon Denshoku Industries Co., Ltd. was used
for the measurement of 20° specular glossiness.

[0187] A: 20° specular glossiness is 100% or more

[0188] B: 20° specular glossiness is 80% or more and less than
100%.

[0189] C: 20° specular glossiness is 60% or more and less than 80%.

[0190] D: 20° specular glossiness is less than 60%.

(Evaluation of Ink Mixing)

[0191] In the recorded image, 50% Duty image was observed by visual
inspection and by using microscope and evaluated based on the following
criteria.

[0192] A: No patchy pattern caused by coalescence of droplets was observed
by visual inspection and few coalescence of each droplets was observed by
using microscope.

[0193] B: Patchy pattern caused by coalescence of droplets was observed
without particular distinction by visual inspection and slight
coalescence of each droplet was observed by using microscope.

[0194] C: Some patchy patterns caused by coalescence of droplets were
observed partly by visual inspection and much coalescence of droplets was
observed by using microscope.

[0195] D: Patchy patterns caused by coalescence of droplets were
apparently observed by visual inspection and they degraded image quality.

(Evaluation of Ejection Stability)

[0196] Under the ambience of 25° C., relative humidity 30%,
evaluation images were printed continuously 8 times, and image 100% Duty
in 8th print was evaluated based on the following criteria.

[0197] A: No image defect was noted.

[0198] B: Slight blur was observed at the start portion of the image (not
more than 2 mm).

[0201] After evaluation images were printed under ambience of 25°
C., relative humidity 30%, nozzle surface was standing for 1 hour without
capping. Subsequently, immediately after maintenance of nozzle was
carried out, evaluation image was printed under ambience of 25°
C., relative humidity 30%, and resulting image was observed and evaluated
based on the following criteria.

[0207] The results described in Table 3 clearly show that each of the inks
of the present invention exhibits excellent performances such as ink
ejection and maintenance property, as well as abrasion resistance,
adhesiveness, glossiness and ink mixing in case of printing onto
polyvinylchloride as recording medium, compared to the comparative inks.

Example 2

Preparation of Pigment Dispersion

[0208] Yellow, Magenta, Cyan, Black Pigment dispersion each was prepared
in the same manner as Example 1, by using carbon black, C.I. Pigment Red
122, C.I. Pigment Yellow 74 as well as C.I. Pigment Blue 15:3.

[0209] Ink sets listed in Table 4 were prepared in the same manner as
Example 1, except for employing Pigment dispersions, Copolymer resin
prepared in Example 1, base for neutralization, organic solvent, and
surfactant as shown in Table 4.

[0210] These ink set were set in the on-demand type inkjet printer used in
Example 1. Then, solid images by 10 cm×10 cm size with 100% Duty
and images with 4 color thin lines overlapped in reticular patterns were
printed to be a recorded image.

[0211] With respect to the resulting evaluation images, abrasion
resistance, adhesiveness, and glossiness were evaluated in the same
manner as Example 1. Further, color bleeding were evaluated based on the
following criteria.

(Evaluation of Color Bleeding)

[0212] Color bleeding is a phenomenon in which ink droplets adjacently
deposited are mixed and a smear is produced in the boundary area of the
image having a different color.

[0213] Images with 4 color thin lines overlapped in reticular patterns
were observed by visual inspection and by using microscope and evaluated
based on the following criteria.

[0219] The results described in Table 5 clearly show that each of the inks
of the present invention exhibits excellent performances such as abrasion
resistance, adhesiveness, glossiness and color bleeding in case of
printing onto polyvinylchloride as recording medium, compared to the
comparative inks.

Example 3

[0220] By using the same ink set used in Example 2, printing was performed
under the same conditions as Example 2. Then, instead of drying after
printing for 24 hours at room temperature, printed images were prepared
by drying for 5 minutes at the temperature described in Table 6
immediately after printing and further drying for 6 hours at room
temperature.

[0221] After printing, back side of t the recorded image was heat-dried by
controlling heater to be the temperature described in Table 6 at a
surface temperature of the recording medium. Herein, the surface
temperature of the recording medium was determined by using Non-contact
thermometer.

[0222] Resulting recorded image were evaluated as follows:

(Abrasion Resistance 2)

[0223] Each of the resulting solid images of 100% Duty was scrubbed by dry
cotton (Kanakin No. 3) with load of 700 g. Abrasion resistance was
evaluated based on the following criteria.

[0224] A: No change was observed in image during frictions of 50 times or
more.

[0225] B: Slight scratch remained but did not affect image density after
frictions of 50 times.

[0226] C: Image density became lowered during frictions between 21 and
less than 50 times.

[0227] D: Image density became lowered during frictions of less than 20
times.

[0228] Ranks A to C were considered to be practically preferable in above
criteria.

(Adhesiveness 2)

[0229] Three cm of adhesive cellophane tape was adhered onto each
resulting solid image of 100% Duty. After that, the tape was quickly
peeled from the layer repeatedly 5 times at the same position, and the
surface state of the image was observed.

[0230] A: No change was observed at the surface state.

[0231] B: Slight trace was remained on the surface.

[0232] C: A part of the image was peeled off and the decrease of color
density was observed.

[0233] D: Most of the image was peeled off and the substrate was observed
as white background.

[0234] Ranks A to C were considered to be practically preferable in above
criteria.

[0236] The results described in Table 6 clearly show that each of the inks
of the present invention exhibits excellent abrasion resistance and
adhesiveness in case of printing onto polyvinylchloride as recording
medium, compared to the comparative inks. Further, performance was more
improved in case of heat-drying than drying only at room temperature.

Example 4

Synthesis of Copolymer Resin

[Synthesis of Copolymer Resin 1: Ammonium Salt]

[0237] Into 500 ml of four-neck flask provided with a mechanical stirrer,
a nitrogen gas directing tube, a condenser, and a dropping funnel, were
loaded 185 g of isopropyl alcohol, and heated to reflux, while bubbling a
nitrogen gas. Into the dropping funnel, 46 g of methyl methacrylate, 39 g
of n-butyl methacrylate, 15 g of methacrylic acid and 0.5 g of
azobisisobutyronitrile (AIBN) as an initiator were dissolved in mixture,
and the mixture was dropped for about 2 hours in a state of heated to
reflux. After finishing dropping, further heated to reflux for 6 hours,
and isopropyl alcohol solution containing 0.05 g of AIBN was dropped for
15 minutes. Thereafter, resulting solution was heated to reflux for
further 5 hours.

[0238] After standing to cool the reaction solution, isopropyl alcohol was
distilled away under reduced pressure. Into the resulting residue, 12.4 g
of 28% ammonia water as a base for alkali neutralization and 553 g of ion
exchanged water were added and followed by heating and mixing to
dissolve. Solid content of resin in this Copolymer resin 1 solution
(ammonia salt) was about 15% by mass.

[0239] Acid value, glass transition temperature and weight average
molecular weight of prepared Copolymer resin 1 were determined based on
the methods described above.

[0243] Copolymer resin 1A being dimethylaminoethanol salt was prepared in
the same manner as preparation of Copolymer resin 1 described above,
except for using 18.2 g of N,N-dimethylaminoethanol instead of 12.4 g of
28% ammonia water.

[0244] Acid value, glass transition temperature and weight average
molecular weight of Copolymer resin 1A were determined based on the
methods described above. Results were listed in Table 7.

[Preparation of Copolymer Resins 2 to 16]

[0245] Copolymer resins 2 to 16 were prepared in the same manner as
preparation of Copolymer resin 1 described above, except for changing
monomer composition as listed in Table 1. Herein, polymerization
conditions were appropriately arranged so as to obtain weight average
molecular weight listed in Table 1.

[0246] Acid value, glass transition temperature and weight average
molecular weight of Copolymer resins 2 to 16 were determined based on the
methods described above. Results were listed in Table 7.

[0258] After mixing above additives, the foresaid solution was shaken for
6 hours by using a paint conditioner filled at a volume ratio of 50% with
zirconia beads of 0.5 mm, followed by eliminating beads, whereby Black
pigment dispersion Bk was prepared.

[0270] Each ink prepared above was evaluated according to the following
methods.

[Image Formation]

[0271] Evaluation was carried out by using an on-demand type inkjet
printer which has 4 lines of piezo type inkjet heads with solution of 720
dpi (herein, dpi refers to as number of dots in 2.54 cm) and an amount of
an minimum ink droplet beings 14 pl. The inkjet printer can warm a medium
from the bottom by a contact type heater, being equipped with a position
for ink idle print and a maintenance unit in a head housing position, and
can perform head cleaning in arbitrary frequency.

[0272] Each ink listed in Table 2 was set in the inkjet head for K ink in
above inkjet printer. Then, onto recording medium of soft vinyl chloride
sheet for solvent inkjet printer, solid image by 10 cm×50 cm size
with 10% to 100% Duty in 10% Duty step were printed by K ink to be a
recorded image. During printing onto the polyvinyl chloride recording
medium, back side of the recording medium was heated by controlling
heater to be at 43° C. at a surface temperature of the recording
medium when image was recorded. After recording, the printed matter was
dried for 24 hours under room temperature (25° C.) and the
recorded images and printing performances were evaluated as follows.

[Condition for Each Evaluation]

[0273] Consequently, following evaluations were carried out.

(Evaluation of Ejection Stability)

[0274] Under the ambience of 20° C., relative humidity 30%,
evaluation images were printed continuously 5 times, and image 100% Duty
in 5th print was evaluated based on the following criteria.

[0275] 3: No image defect was noted.

[0276] 2: Slight blur was observed at the start portion of the image
(several mm).

[0278] After evaluation images were printed under ambience of 20°
C., relative humidity 30%, then, nozzle surface was standing for 1 hour
without capping. Subsequently, immediately after maintenance of nozzle
was carried out by employing blade type maintenance unit, solid image was
once printed under ambience of 20° C., relative humidity 30%, and
resulting image was observed by visual inspection and maintenance
property was evaluated based on the following criteria.

[0281] 1: In resulting image, much image defects by maintenance failure
were observed.

(Evaluation of Ink Mixing Resistance)

[0282] Above image was once printed under ambience of 30° C.,
relative humidity 60%. In the recorded image, 10% to 100% Duty images in
10% Duty step were observed by visual inspection and by using microscope
in view of coalescence of droplets (ink mixing) and Ink mixing resistance
was evaluated based on the following criteria.

[0283] 3: No patchy pattern caused by coalescence of droplets was observed
by visual inspection and few coalescence of each droplets was observed by
using microscope.

[0284] 2: Slight patchy patterns caused by coalescence of droplets were
observed by visual inspection and slight coalescence of droplets was
observed by using microscope.

[0285] 1: Patchy patterns caused by coalescence of droplets were
apparently observed by visual inspection and they degraded image quality.

(Evaluation of Glossiness)

[0286] For the solid image portion having 100% Duty of the recorded image,
20° specular glossiness was measured. A declination glossmeter
VGS-10001DP supplied from Nippon Denshoku Industries Co., Ltd. was used
for the measurement of 20° specular glossiness.

[0287] Glossiness was evaluated based on the following criteria from the
resulting 20° specular glossiness.

[0288] 5: 20° specular glossiness is 100% or more

[0289] 4: 20° specular glossiness is 85% or more and less than 100%

[0290] 3: 20° specular glossiness is 55% or more and less than 85%

[0291] 2: 20° specular glossiness is 30% or more and less than 55%

[0292] 1: 20° specular glossiness is less than 30%.

(Abrasion Resistance)

[0293] Each of the resulting solid images was scrubbed with load of 9N by
dry cotton (Kanakin No. 3) and state of scratch on image surface and peel
of layer were visually observed and abrasion resistance was evaluated
based on the following criteria.

[0294] 5: No scratch was observed on the image after 30 times of
reciprocating friction.

[0295] 4: No scratch was observed on image after 10 times of reciprocating
friction, however slight scratch was observed on the surface after 30
times of reciprocating friction.

[0296] 3: Scratch was observed on the surface after 10 times of
reciprocating friction, and image layer was peeled off after 30 times of
reciprocating friction.

[0297] 2: Image layer was peeled off after 10 times of reciprocating
friction.

[0298] 1: Image layer was peeled off after 5 times of reciprocating
friction.

(Adhesiveness)

[0299] Cross-cut adhesiveness test was performed on the image prepared by
the above method. Cross-cut adhesiveness test was based on a method
defined in JIS K 5600 (General test method for Paint), the 5th part
(Mechanical performance of coating layer), the 6th section
(Adhesiveness: Cross-cut method). Eleven cutting lines orthogonal
oriented each other at 1 mm intervals were provided onto the resulting
image. After adhesive tape of cellophane (LP-24 produced by Nichiban) was
pressed to adhere onto cross-cut image, the tape was quickly peeled from
the layer, and the state of peel off was determined by visual inspection
to evaluate adhesiveness based on the following criteria.

[0305] The results described in Table 9 clearly show that each of the inks
of the present invention exhibits excellent ejection stability and
maintenance property; and also formed image has excellent ink mixing
resistance, glossiness, and adhesiveness to substrate (polyvinylchloride
sheet), compared to the comparative inks.

Example 5

Preparation of Ink

[Preparation of Inks 19 to 27]

[0306] Inks 19 to 27 were prepared by the same manner as the preparation
of Ink 1 described in Example 4, except for changing species and content
of copolymer resin, solvent compositions, species and content of
surfactant as well as species and content of alkanol amines as listed in
Table 10.

[0316] Images were formed by using each ink prepared above and evaluated
according to the following methods.

[Image Formation]

[0317] Inks listed in Table 11 were set in the inkjet head for K ink in
the inkjet printer described in Example 4. Then, onto recording medium of
soft vinyl chloride sheet for solvent inkjet printer, solid image by 50
cm×50 cm size with 100% Duty and images with thin lines in grid
patterns were printed by each ink to be a recorded image. Back side of
the recording medium was heated by controlling heater to be at 47°
C. at a surface temperature of the recording medium when image was
recorded.

[Condition for Each Evaluation]

[0318] Consequently, ejection stability 2 and drying performance of formed
image for each ink were evaluated based on the following method. Further,
evaluation of abrasion resistance was performed in the same manner as
Example 4. As for evaluation other than drying performance, the printed
matter was dried for 24 hours under room temperature (25° C.)
after recording.

(Evaluation of Ejection Stability 2)

[0319] Under the above printing conditions, evaluation images were printed
continuously 10 times, and 100% Duty image and image of thin lines of the
last print was observed by visual inspection and Ejection stability 2 was
evaluated based on the following criteria.

[0320] 3: No image defect was noted in solid image and dots were clearly
reproduced at thin line. Also no distorted image was noted.

[0322] 1: Image defects were observed partially in the solid image and
distortion was apparently observed in thin line.

(Evaluation of Drying Performance)

[0323] Image immediately after printing was dried by employing warm air
dryer of 60° C. After drying, cotton cloth was contacted on the
image and a time until ink did not contaminate cotton was determined and
was employed as criteria of drying performance. The smaller the number
refers more excellent drying performance.

[0325] The results described in Table 11 clearly show that the inks of the
present invention enhances ejection stability 2 and abrasion resistance
without affecting on drying performance by employing alkanol amines in
combination.

Example 6

Preparation of Ink Set

[Preparation of Inks 28 to 35]

[0326] Inks 28 to 35 were prepared by the same manner as the preparation
of Ink 1 described in Example 4, except for changing species of pigment
dispersion and content of pigment, species and content of copolymer resin
as listed in Table 12.

[0327] Each of additives described by abbreviation in Table 12 was
detailed as follows:

[0328] Y: Yellow pigment dispersion Y

[0329] M: Magenta pigment dispersion M

[0330] C: Cyan pigment dispersion C

[0331] Bk: Black pigment dispersion Bk

[Ink Set 1]

[0332] Inks 28(Y), 29(M), 30C and 31(Bk) prepared above were combined to
be Ink set 1.

[Ink Set 2]

[0333] Inks 32(Y), 33(M), 34C and 35(Bk) prepared above were combined to
be Ink set 2.

<<Evaluation of Ink>>

[0334] Images were formed by using each ink prepared above and evaluated
according to the following methods.

[Image Formation]

[0335] Each ink was set in the inkjet heads for Y ink, M ink, C ink and Bk
ink respectively in the inkjet printer described in Example 4. Then, onto
recording medium, high-definition color digital standard image by 10
cm×10 cm size was printed in the same manner as the method
described in Example 4.

[Condition for Each Evaluation]

[0336] Consequently, ink mixing resistance 2 was evaluated based on the
following method. Further, evaluations of glossiness and abrasion
resistance were performed in the same manner as Example 4.

(Evaluation of Ink Mixing Resistance 2)

[0337] In the high-definition color digital standard image, color bleeding
between each color or patchy pattern caused by beading at the secondary
color which has much ink amount was observed by visual inspection. Ink
mixing resistance 2 was evaluated based on the following criteria.

[0338] 3: No color bleeding and no patchy pattern was observed. High
quality print was achieved.

[0342] The results described in Table 13 clearly show that the inks of the
present invention exhibits excellent ink mixing resistance, glossiness
and abrasion resistance, compared to the comparative inks.

Example 7

[0343] By employing ink set in Example 6, in the same manner as Example 6,
printing was performed. Instead of drying after printing for 24 hours at
room temperature (25° C.), printed image was formed by drying for
3 minutes at the temperature listed in Tables 14 and 15 immediately after
printing, followed by further drying for 6 hours at room temperature.
Herein, image after printing was dried by employing warm air dryer.

[0344] Resulting recorded image were evaluated as follows:

(Abrasion Resistance 2)

[0345] Each of the resulting solid images of 100% Duty was scrubbed by dry
cotton (Kanakin No. 3) with load of 18N. Scratch on the surface of image
and peeling off of layer was visually observed and abrasion resistance
was evaluated based on the following criteria.

[0346] 5: No scratch was observed on the image after 30 times of
reciprocating friction.

[0347] 4: No scratch was observed on image after 10 times of reciprocating
friction, however slight scratch was observed on the surface after 30
times of reciprocating friction.

[0348] 3: Scratch was observed on the surface after 10 times of
reciprocating friction, and image layer was peeled off after 30 times of
reciprocating friction.

[0349] 2: Image layer was peeled off after 10 times of reciprocating
friction.

[0350] 1: Image layer was peeled off after 5 times of reciprocating
friction.

[0351] Ranks 3 to 5 were considered to be practically preferable in above
criteria.

(Adhesiveness 2)

[0352] Three cm of adhesive cellophane tape was adhered onto each
resulting solid image of 100% Duty. After that, the tape was quickly
peeled from the layer repeatedly 5 times at the same position, and the
surface state of the image was evaluated.

[0353] 4: No change was observed at the surface state.

[0354] 3: Slight trace was remained on the surface.

[0355] 2: A part of the image was peeled off and the decrease of color
density was observed.

[0356] 1: Most of the image was peeled off and the substrate was observed
as white background.

[0357] Ranks 3 to 4 were considered to be practically preferable in above
criteria.

[0359] The results described in Tables 14 and 15 clearly show that the ink
set of the present invention exhibits excellent performances such as
abrasion resistance and adhesiveness, in case of printing onto soft
polyvinylchloride as recording medium, compared to the comparative inks.
Further, these performances were more enhanced in the case of drying with
heating than the case of drying at room temperature.